Method for making epitaxial structure

ABSTRACT

A method for making an epitaxial structure is provided. The method includes the following steps. A substrate is provided. The substrate has an epitaxial growth surface for growing epitaxial layer. A first carbon nanotube layer is placed on the epitaxial growth surface. A first epitaxial layer is epitaxially grown on the epitaxial growth surface. A second carbon nanotube layer is placed on the first epitaxial layer. A second epitaxial layer is epitaxially grown on the first epitaxial layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/273,252, filed Oct. 14, 2011, entitled,“EPITAXIAL STRUCTURE,” which claims all benefits accruing under 35U.S.C. §119 from China Patent Applications: Application No.201110005809.X, filed on Jan. 12, 2011; Application No. 201110025832.5,filed on Jan. 24, 2011; Application No. 201110025768.0, filed on Jan.24, 2011; Application No. 201110025710.6, filed on Jan. 24, 2011;Application No. 201110077488.4, filed on Mar. 29, 2011; Application No.201110076893.4, filed on Mar. 29, 2011; Application No. 201110076876.0,filed on Mar. 29, 2011; Application No. 201110076867.1, filed on Mar.29, 2011; Application No. 201110076886.4, filed on Mar. 29, 2011;Application No. 201110076887.9, filed on Mar. 29, 2011; Application No.201110076901.5, filed on Mar. 29, 2011; Application No. 201110076903.4,filed on Mar. 29, 2011; Application No. 201110095149.9, filed on Apr.15, 2011; in the China Intellectual Property Office, disclosures ofwhich are incorporated herein by references.

BACKGROUND

1. Technical Field

The present disclosure relates to epitaxial structures and methods formaking the same.

2. Description of Related Art

Light emitting devices such as light emitting diodes (LEDs) based ongroup III-V nitride semiconductors such as gallium nitride (GaN) havebeen put into practice.

Since wide GaN substrate cannot be produced, the LEDs have been producedon a heteroepitaxial substrate such as sapphire. The use of sapphiresubstrate is problematic due to lattice mismatch and thermal expansionmismatch between GaN and the sapphire substrate. One consequence ofthermal expansion mismatch is bowing of the GaN/sapphire substratestructure, which leads to cracking and difficulty in fabricating deviceswith small feature sizes. A solution for this is to form a plurality ofgrooves on the surface of the sapphire substrate by lithography oretching before growing the GaN layer. However, the process oflithography and etching is complex, high in cost, and will pollute thesapphire substrate.

What is needed, therefore, is to provide a method for solving theproblem discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making anepitaxial structure.

FIG. 2 is a base for growing an epitaxial structure of one embodiment,wherein the base includes a plurality of carbon nanotubes located on asubstrate and extending along the same direction.

FIG. 3 is a base for growing an epitaxial structure of one embodiment,wherein the base includes a plurality of carbon nanotubes located on asubstrate and extending along two directions perpendicular with eachother.

FIG. 4 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film.

FIG. 5 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 4.

FIG. 6 is an SEM image of cross-stacked drawn carbon nanotube films.

FIG. 7 is an SEM image of a pressed carbon nanotube film.

FIG. 8 is an SEM image of a flocculated carbon nanotube film.

FIG. 9 is an SEM image of an untwisted carbon nanotube wire.

FIG. 10 is an SEM image of a twisted carbon nanotube wire.

FIG. 11 is a process of growing a first epitaxial layer on a substrate.

FIG. 12 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 1.

FIG. 13 is a schematic, cross-sectional view, along a line XIII-XIII ofFIG. 12.

FIG. 14 is a schematic view of another embodiment of an epitaxialstructure fabricated in the method of FIG. 1.

FIG. 15 is a schematic view of another embodiment of an epitaxialstructure fabricated in the method of FIG. 1.

FIG. 16 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 17 is a process of growing a second epitaxial layer on the firstepitaxial layer of FIG. 1.

FIG. 18 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 16.

FIG. 19 is a schematic, cross-sectional view, along a line XIX-XIX ofFIG. 18.

FIG. 20 is a schematic view of another embodiment of an epitaxialstructure fabricated in the method of FIG. 16.

FIG. 21 is a schematic view of another embodiment of an epitaxialstructure fabricated in the method of FIG. 16.

FIG. 22 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 23 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 24 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 25 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 24.

FIG. 26 is a schematic, cross-sectional view, along a line XXVI-XXVI ofFIG. 25.

FIG. 27 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 28 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 29 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 30 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 29.

FIG. 31 is a schematic, cross-sectional view, along a line XXXI-XXXI ofFIG. 30.

FIG. 32 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 33 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 32.

FIG. 34 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 35 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 34.

FIG. 36 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 37 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 36.

FIG. 38 is a flowchart of another embodiment of a method for making anepitaxial structure.

FIG. 39 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 38.

FIG. 40 is a schematic view of one embodiment of an epitaxial structurefabricated in the method of FIG. 38.

FIG. 41 is an SEM image of a cross-section of the epitaxial structurefabricated in example 1.

FIG. 42 is a transmission electron microscopy (TEM) image of a crosssection of the epitaxial structure fabricated in example 1.

FIG. 43 is an SEM image of a cross section of the epitaxial structurefabricated in example 2.

FIG. 44 is a TEM image of a cross section the epitaxial structurefabricated in example 2.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present epitaxial structures and methods formaking the same.

Referring to FIG. 1, a method for making an epitaxial structure 10 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101; and

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101.

In step (10), the epitaxial growth surface 101 can be used to grow thefirst epitaxial layer 104. The epitaxial growth surface 101 is a cleanand smooth surface. The substrate 100 can be a single-layer structure ora multi-layer structure. If the substrate 100 is a single-layerstructure, the substrate 100 can be a single crystal structure having acrystal face used as the epitaxial growth surface 101. If the substrate100 is a multi-layer structure, the substrate 100 should include atleast one layer having the crystal face. The material of the substrate100 can be GaAs, GaN, AlN, Si, SOI (silicon on insulator), SiC, MgO,ZnO, LiGaO₂, LiAlO₂, or Al₂O₃. The material of the substrate 100 can beselected according to the material of the first epitaxial layer 104. Thefirst epitaxial layer 104 and the substrate 100 should have a smalllattice mismatch and a thermal expansion mismatch. The size, thickness,and shape of the substrate 100 can be selected according to need. In oneembodiment, the substrate 100 is a sapphire substrate.

In step (20), a base 100 a for growing the first epitaxial layer 104 isobtained as shown in FIGS. 2 and 3. The base 100 a includes a substrate100 having an epitaxial growth surface 101 and a first carbon nanotubelayer 102 located thereon. The base 100 a can be used to grow the firstepitaxial layer 104 directly.

The first carbon nanotube layer 102 includes a plurality of carbonnanotubes. The carbon nanotubes in the first carbon nanotube layer 102can be single-walled, double-walled, or multi-walled carbon nanotubes.The length and diameter of the carbon nanotubes can be selectedaccording to need. The thickness of the first carbon nanotube layer 102can be in a range from about 1 nanometer to about 100 micrometers. Forexample, the thickness of the first carbon nanotube layer 102 can beabout 10 nanometers, 100 nanometers, 200 nanometers, 1 micrometer, 10micrometers, or 50 micrometers. The first carbon nanotube layer 102forms a pattern, therefore, part of the epitaxial growth surface 101 canbe exposed from the patterned first carbon nanotube layer 102 after thefirst carbon nanotube layer 102 is placed on the epitaxial growthsurface 101. Thus, the first epitaxial layer 104 can grow from theexposed epitaxial growth surface 101.

The patterned first carbon nanotube layer 102 defines a plurality offirst apertures 105. The first apertures 105 can be dispersed uniformly.The first aperture 105 extends throughout the first carbon nanotubelayer 102 along the thickness direction thereof. The first aperture 105can be a hole defined by several adjacent carbon nanotubes, or a gapdefined by two substantially parallel carbon nanotubes and extendingalong axial direction of the carbon nanotubes. The hole shaped firstaperture 105 and the gap shaped first aperture 105 can exist in thepatterned first carbon nanotube layer 102 at the same time. Hereafter,the size of the first aperture 105 is the diameter of the hole or widthof the gap. The sizes of the first apertures 105 can be different. Theaverage size of the first apertures 105 can be in a range from about 10nanometers to about 500 micrometers. For example, the sizes of the firstapertures 105 can be about 50 nanometers, 100 nanometers, 500nanometers, 1 micrometer, 10 micrometers, 80 micrometers, or 120micrometers. The smaller the sizes of the first apertures 105, the lessdislocation defects will occur during the process of growing the firstepitaxial layer 104. In one embodiment, the sizes of the first apertures105 are in a range from about 10 nanometers to about 10 micrometers. Adutyfactor of the first carbon nanotube layer 102 is an area ratiobetween the sheltered epitaxial growth surface 101 and the exposedepitaxial growth surface 101. The dutyfactor of the first carbonnanotube layer 102 can be in a range from about 1:100 to about 100:1.For example, the dutyfactor of the first carbon nanotube layer 102 canbe about 1:10, 1:2, 1:4, 4:1, 2:1, or 10:1. In one embodiment, thedutyfactor of the first carbon nanotube layer 102 is in a range fromabout 1:4 to about 4:1.

The carbon nanotubes of the first carbon nanotube layer 102 can beorderly arranged to form an ordered carbon nanotube structure ordisorderly arranged to form a disordered carbon nanotube structure. Theterm ‘disordered carbon nanotube structure’ includes, but is not limitedto, a structure wherein the carbon nanotubes are arranged along manydifferent directions, and the aligning directions of the carbonnanotubes are random. The number of the carbon nanotubes arranged alongeach different direction can be almost the same (e.g. uniformlydisordered). The disordered carbon nanotube structure can be isotropic.The carbon nanotubes in the disordered carbon nanotube structure can beentangled with each other. The term ‘ordered carbon nanotube structure’includes, but is not limited to, a structure wherein the carbonnanotubes are arranged in a consistently systematic manner, e.g., thecarbon nanotubes are arranged approximately along a same directionand/or have two or more sections within each of which the carbonnanotubes are arranged approximately along a same direction (differentsections can have different directions).

In one embodiment, the carbon nanotubes in the first carbon nanotubelayer 102 are arranged to extend along the direction substantiallyparallel to the surface of the first carbon nanotube layer 102 so thatit is easy to obtain a pattern having greater light transmission. Afterplacement on the epitaxial growth surface 101, the carbon nanotubes inthe first carbon nanotube layer 102 can be arranged to extend along thedirection substantially parallel to the epitaxial growth surface 101.Referring to FIG. 2, a majority of the carbon nanotubes in the firstcarbon nanotube layer 102 are arranged to extend along the samedirection. Referring to FIG. 3, some of the carbon nanotubes in thefirst carbon nanotube layer 102 are arranged to extend along a firstdirection, and the rest of the carbon nanotubes in the first carbonnanotube layer 102 are arranged to extend along a second direction,substantially perpendicular to the first direction. The carbon nanotubesin the ordered carbon nanotube structure can also be arranged to extendalong the crystallographic orientation of the substrate 100 or along adirection which forms an angle with the crystallographic orientation ofthe substrate 100.

The first carbon nanotube layer 102 can be formed on the epitaxialgrowth surface 101 by chemical vapor deposition (CVD), transfer printinga preformed carbon nanotube film, or filtering and depositing a carbonnanotube suspension. In one embodiment, the first carbon nanotube layer102 is a free-standing structure and can be drawn from a carbon nanotubearray. The term “free-standing structure” means that the first carbonnanotube layer 102 can sustain the weight of itself when it is hoistedby a portion thereof without any significant damage to its structuralintegrity. Thus, the first carbon nanotube layer 102 can be suspended bytwo spaced supports. The free-standing first carbon nanotube layer 102can be laid on the epitaxial growth surface 101 directly and easily.

The first carbon nanotube layer 102 can be a substantially purestructure of carbon nanotubes, with few impurities and chemicalfunctional groups. The first carbon nanotube layer 102 can also be acomposite including a carbon nanotube matrix and non-carbon nanotubematerials. The non-carbon nanotube materials can be graphite, graphene,silicon carbide, boron nitride, silicon nitride, silicon dioxide,diamond, amorphous carbon, metal carbides, metal oxides, or metalnitrides. The non-carbon nanotube materials can be coated on the carbonnanotubes of the first carbon nanotube layer 102 or filled in the firstaperture 105. In one embodiment, the non-carbon nanotube materials arecoated on the carbon nanotubes of the first carbon nanotube layer 102 sothat the carbon nanotubes can have greater diameters and the firstapertures 105 can be smaller. The non-carbon nanotube materials can bedeposited on the carbon nanotubes of the first carbon nanotube layer 102by CVD or physical vapor deposition (PVD), such as sputtering.

Furthermore, the first carbon nanotube layer 102 can be treated with anorganic solvent after being placed on the epitaxial growth surface 101so that the first carbon nanotube layer 102 can be attached on theepitaxial growth surface 101 firmly. Specifically, the organic solventcan be applied to entire surface of the first carbon nanotube layer 102or the entire first carbon nanotube layer 102 can be immersed in anorganic solvent. The organic solvent can be volatile, such as ethanol,methanol, acetone, dichloroethane, chloroform, or mixtures thereof. Inone embodiment, the organic solvent is ethanol.

The first carbon nanotube layer 102 can include at least one carbonnanotube film, at least one carbon nanotube wire, or combinationthereof. In one embodiment, the first carbon nanotube layer 102 caninclude a single carbon nanotube film or two or more carbon nanotubefilms stacked together. Thus, the thickness of the first carbon nanotubelayer 102 can be controlled by the number of the stacked carbon nanotubefilms. The number of the stacked carbon nanotube films can be in a rangefrom about 2 to about 100. For example, the number of the stacked carbonnanotube films can be 10, 30, or 50. In one embodiment, the first carbonnanotube layer 102 can include a layer of parallel and spaced carbonnanotube wires. Also, the first carbon nanotube layer 102 can include aplurality of carbon nanotube wires crossed or weaved together to form acarbon nanotube net. The distance between two adjacent parallel andspaced carbon nanotube wires can be in a range from about 0.1micrometers to about 200 micrometers. In one embodiment, the distancebetween two adjacent parallel and spaced carbon nanotube wires is in arange from about 10 micrometers to about 100 micrometers. The gapbetween two adjacent substantially parallel carbon nanotube wires isdefined as the first aperture 105. The size of the first aperture 105can be controlled by controlling the distance between two adjacentparallel and spaced carbon nanotube wires. The length of the gap betweentwo adjacent parallel carbon nanotube wires can be equal to the lengthof the carbon nanotube wire. It is understood that any carbon nanotubestructure described can be used with all embodiments.

In one embodiment, the first carbon nanotube layer 102 includes at leastone drawn carbon nanotube film. A drawn carbon nanotube film can bedrawn from a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 4 to 5, each drawn carbonnanotube film includes a plurality of successively oriented carbonnanotube segments 143 joined end-to-end by van der Waals attractiveforce therebetween. Each carbon nanotube segment 143 includes aplurality of carbon nanotubes 145 parallel to each other, and combinedby van der Waals attractive force therebetween. As can be seen in FIG.4, some variations can occur in the drawn carbon nanotube film. Thecarbon nanotubes 145 in the drawn carbon nanotube film are orientedalong a preferred orientation. The drawn carbon nanotube film can betreated with an organic solvent to increase the mechanical strength andtoughness and reduce the coefficient of friction of the drawn carbonnanotube film. A thickness of the drawn carbon nanotube film can rangefrom about 0.5 nanometers to about 100 micrometers. The drawn carbonnanotube film can be attached to the epitaxial growth surface 101directly.

The first carbon nanotube layer 102 can include at least two stackeddrawn carbon nanotube films. In other embodiments, the first carbonnanotube layer 102 can include two or more coplanar carbon nanotubefilms, and can include layers of coplanar carbon nanotube films.Additionally, when the carbon nanotubes in the carbon nanotube film arealigned along one preferred orientation (e.g., the drawn carbon nanotubefilm), an angle can exist between the orientation of carbon nanotubes inadjacent films, whether stacked or adjacent. Adjacent carbon nanotubefilms can be combined by only the van der Waals attractive forcetherebetween. An angle between the aligned directions of the carbonnanotubes in two adjacent carbon nanotube films can range from about 0degrees to about 90 degrees. When the angle between the aligneddirections of the carbon nanotubes in adjacent stacked drawn carbonnanotube films is larger than 0 degrees, a plurality of micropores isdefined by the first carbon nanotube layer 102. Referring to FIG. 6, thefirst carbon nanotube layer 102 is shown with the aligned directions ofthe carbon nanotubes between adjacent stacked drawn carbon nanotubefilms at 90 degrees. Stacking the carbon nanotube films will also add tothe structural integrity of the first carbon nanotube layer 102.

A step of heating the drawn carbon nanotube film can be performed todecrease the thickness of the drawn carbon nanotube film. The drawncarbon nanotube film can be partially heated by a laser or microwave.The thickness of the drawn carbon nanotube film can be reduced becausesome of the carbon nanotubes will be oxidized. In one embodiment, thedrawn carbon nanotube film is irradiated by a laser device in anatmosphere comprising of oxygen therein. The power density of the laseris greater than 0.1×10⁴ watts per square meter. The drawn carbonnanotube film can be heated by fixing the drawn carbon nanotube film andmoving the laser device at a substantially uniform speed to irradiatethe drawn carbon nanotube film. When the laser irradiates the drawncarbon nanotube film, the laser is focused on the surface of the drawncarbon nanotube film to form a laser spot. The diameter of the laserspot ranges from about 1 micron to about 5 millimeters. In oneembodiment, the laser device is carbon dioxide laser device. The powerof the laser device is about 30 watts. The wavelength of the laser isabout 10.6 micrometers. The diameter of the laser spot is about 3millimeters. The velocity of the laser movement is less than 10millimeters per second. The power density of the laser is 0.053×10¹²watts per square meter.

In another embodiment, the first carbon nanotube layer 102 can include apressed carbon nanotube film. Referring to FIG. 7, the pressed carbonnanotube film can be a free-standing carbon nanotube film. The carbonnanotubes in the pressed carbon nanotube film are arranged along a samedirection or arranged along different directions. The carbon nanotubesin the pressed carbon nanotube film can rest upon each other. Adjacentcarbon nanotubes are attracted to each other and combined by van derWaals attractive force. An angle between a primary alignment directionof the carbon nanotubes and a surface of the pressed carbon nanotubefilm is about 0 degrees to approximately 15 degrees. The greater thepressure applied, the smaller the angle formed. If the carbon nanotubesin the pressed carbon nanotube film are arranged along differentdirections, the first carbon nanotube layer 102 can be isotropic.

In another embodiment, the first carbon nanotube layer 102 includes aflocculated carbon nanotube film. Referring to FIG. 8, the flocculatedcarbon nanotube film can include a plurality of long, curved, disorderedcarbon nanotubes entangled with each other. Furthermore, the flocculatedcarbon nanotube film can be isotropic. The carbon nanotubes can besubstantially uniformly dispersed in the carbon nanotube film. Adjacentcarbon nanotubes are acted upon by van der Waals attractive force toform an entangled structure with micropores defined therein. Sizes ofthe micropores can be less than 10 micrometers. The porous nature of theflocculated carbon nanotube film will increase the specific surface areaof the first carbon nanotube layer 102. Further, due to the carbonnanotubes in the first carbon nanotube layer 102 being entangled witheach other, the first carbon nanotube layer 102 employing theflocculated carbon nanotube film has excellent durability, and can befashioned into desired shapes with a low risk to the integrity of thefirst carbon nanotube layer 102. The flocculated carbon nanotube film,in some embodiments, is free-standing due to the carbon nanotubes beingentangled and adhered together by van der Waals attractive forcetherebetween.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into an untwisted carbon nanotube wire.Referring to FIG. 9, the untwisted carbon nanotube wire includes aplurality of carbon nanotubes substantially oriented along a samedirection (i.e., a direction along the length of the untwisted carbonnanotube wire). The carbon nanotubes are substantially parallel to theaxis of the untwisted carbon nanotube wire. More specifically, theuntwisted carbon nanotube wire includes a plurality of successive carbonnanotube segments joined end to end by van der Waals attractive forcetherebetween. Each carbon nanotube segment includes a plurality ofcarbon nanotubes substantially parallel to each other, and combined byvan der Waals attractive force therebetween. The carbon nanotubesegments can vary in width, thickness, uniformity, and shape. The lengthof the untwisted carbon nanotube wire can be arbitrarily set as desired.A diameter of the untwisted carbon nanotube wire ranges from about 0.5nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. Referring to FIG.10, the twisted carbon nanotube wire includes a plurality of carbonnanotubes helically oriented around an axial direction of the twistedcarbon nanotube wire. More specifically, the twisted carbon nanotubewire includes a plurality of successive carbon nanotube segments joinedend to end by van der Waals attractive force therebetween. Each carbonnanotube segment includes a plurality of carbon nanotubes parallel toeach other, and combined by van der Waals attractive force therebetween.The length of the carbon nanotube wire can be set as desired. A diameterof the twisted carbon nanotube wire can be from about 0.5 nanometers toabout 100 micrometers. Further, the twisted carbon nanotube wire can betreated with a volatile organic solvent after being twisted to bundlethe adjacent paralleled carbon nanotubes together. The specific surfacearea of the twisted carbon nanotube wire will decrease, while thedensity and strength of the twisted carbon nanotube wire will increase.

The first carbon nanotube layer 102 can be used as a mask for growingthe first epitaxial layer 104. The mask is the first carbon nanotubelayer 102 sheltering a part of the epitaxial growth surface 101 andexposing another part of the epitaxial growth surface 101. Thus, thefirst epitaxial layer 104 can grow from the exposed epitaxial growthsurface 101. The first carbon nanotube layer 102 can form a patternedmask on the epitaxial growth surface 101 because the first carbonnanotube layer 102 defines a plurality of first apertures 105. Comparedto lithography or etching, the method of forming a first carbon nanotubelayer 102 as mask is simple, low in cost, and will not pollute thesubstrate 100.

In step (30), the first epitaxial layer 104 can be grown by a methodsuch as molecular beam epitaxy, chemical beam epitaxy, reduced pressureepitaxy, low temperature epitaxy, select epitaxy, liquid phasedeposition epitaxy, metal organic vapor phase epitaxy, ultra-high vacuumchemical vapor deposition, hydride vapor phase epitaxy, or metal organicchemical vapor deposition (MOCVD).

The first epitaxial layer 104 is a single crystal layer grown on theepitaxial growth surface 101 by epitaxy growth method. The material ofthe first epitaxial layer 104 can be the same as or different from thematerial of the substrate 100. If the first epitaxial layer 104 and thesubstrate 100 are the same material, the first epitaxial layer 104 iscalled a homogeneous epitaxial layer. If the first epitaxial layer 104and the substrate 100 have different material, the first epitaxial layer104 is called a heteroepitaxial epitaxial layer. The material of thefirst epitaxial layer 104 can be semiconductor, metal or alloy. Thesemiconductor can be Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP,AlAs, AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN,AlInN, GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn, or GaP:N. The metal canbe aluminum, platinum, copper, or silver. The alloy can be MnGa, CoMnGa,or Co₂MnGa. The thickness of the first epitaxial layer 104 can beprepared according to need. The thickness of the first epitaxial layer104 can be in a range from about 100 nanometers to about 500micrometers. For example, the thickness of the first epitaxial layer 104can be about 200 nanometers, 500 nanometers, 1 micrometer, 2micrometers, 5 micrometers, 10 micrometers, or 50 micrometers.

Referring to FIG. 11, step (30) includes the following substeps:

step (301), nucleating on the epitaxial growth surface 101 and growing aplurality of epitaxial crystal grains 1042 along the directionsubstantially perpendicular to the epitaxial growth surface 101;

step (302), forming a continuous epitaxial film 1044 by making theepitaxial crystal grains 1042 grow along the direction substantiallyparallel to the epitaxial growth surface 101; and

step (303), forming the first epitaxial layer 104 by making theepitaxial film 1044 grow along the direction substantially perpendicularto the epitaxial growth surface 101.

In step (301), the epitaxial crystal grains 1042 grow from the exposedpart of the epitaxial growth surface 101 and through the first apertures105. The process of the epitaxial crystal grains 1042 growing along thedirection substantially perpendicular to the epitaxial growth surface101 is called vertical epitaxial growth.

In step (302), the epitaxial crystal grains 1042 are joined together toform an integral structure (the epitaxial film 1044) to cover the firstcarbon nanotube layer 102. The epitaxial crystal grains 1042 grow andform a plurality of first caves 103 to enclose the carbon nanotubes ofthe first carbon nanotube layer 102. The inner wall of the first caves103 can be in contact with the carbon nanotubes or spaced from thecarbon nanotubes, depending on whether the material of the epitaxialfilm 1044 and the carbon nanotubes have mutual infiltration. Thus, theepitaxial film 1044 defines a patterned depression on the surfaceadjacent to the epitaxial growth surface 101. The patterned depressionis related to the patterned first carbon nanotube layer 102. If thefirst carbon nanotube layer 102 includes a layer of parallel and spacedcarbon nanotube wires, the patterned depression is a plurality ofparallel and spaced grooves. If the first carbon nanotube layer 102includes a plurality of carbon nanotube wires crossed or weaved togetherto form a carbon nanotube net, the patterned depression is a groovenetwork including a plurality of intersected grooves. The first carbonnanotube layer 102 can prevent lattice dislocation between the epitaxialcrystal grains 1042 and the substrate 100 from growing. The process ofepitaxial crystal grains 1042 growing along the direction substantiallyparallel to the epitaxial growth surface 101 is called lateral epitaxialgrowth.

In step (303), the first epitaxial layer 104 is obtained by growing fora long duration of time. Because the first carbon nanotube layer 102 canprevent the lattice dislocation between the epitaxial crystal grains1042 and the substrate 100 from growing in step (302), the firstepitaxial layer 104 has fewer defects therein.

Referring to FIGS. 12 and 13, an epitaxial structure 10 provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,and a first epitaxial layer 104. The substrate 100 has an epitaxialgrowth surface 101. The first carbon nanotube layer 102 is located onthe epitaxial growth surface 101 and defines a plurality of firstapertures 105. The first epitaxial layer 104 is located on the firstcarbon nanotube layer 102 and contacts the epitaxial growth surface 101through the first apertures 105. The first epitaxial layer 104 defines aplurality of first caves 103 adjacent to and oriented to the epitaxialgrowth surface 101. The first caves 103 can be blind holes or grooves.The first caves 103 and the epitaxial growth surface 101 cooperativelyform a sealed chamber to receive the first carbon nanotube layer 102therein. The inner wall of the first caves 103 can be spaced from thecarbon nanotubes of the first carbon nanotube layer 102. In oneembodiment, the first carbon nanotube layer 102 includes a drawn carbonnanotube film as shown in FIG. 12. In another embodiment, the firstcarbon nanotube layer 102 of an epitaxial structure 10 a includes alayer of parallel and spaced carbon nanotube wires as shown in FIG. 14.In another embodiment, the first carbon nanotube layer 102 of anepitaxial structure 10 b includes a plurality of carbon nanotube wirescrossed or weaved together to form a carbon nanotube net as shown inFIG. 15.

Referring to FIG. 16, a method for making an epitaxial structure 10 c ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101;

step (40), placing a second carbon nanotube layer 107 on a surface 106of the first epitaxial layer 104; and

step (50), epitaxially growing a second epitaxial layer 109 on the firstepitaxial layer 104.

The method for making an epitaxial structure 10 c is similar to themethod for making the epitaxial structure 10 described above exceptadditional steps (40) and (50).

In step (40), the second carbon nanotube layer 107 is the same as thefirst carbon nanotube layer 102. The second carbon nanotube layer 107defines a plurality of second apertures 108. In one embodiment, thesecond carbon nanotube layer 107 is a layer of parallel and spacedcarbon nanotube wires. The second carbon nanotube layer 107 can beplaced on the surface 106 of the first epitaxial layer 104 directly. Thesurface 106 can be used to grow the epitaxial layer.

In step (50), the method for epitaxially growing the second epitaxiallayer 109 is the same as the method for epitaxially growing the firstepitaxial layer 104. The material of the second epitaxial layer 109 andthe material of the first epitaxial layer 104 can be the same. Thesecond epitaxial layer 109 has improved quality because the firstepitaxial layer 104 has less defects therein.

Referring to FIG. 17, step (50) is similar to step (30) and includesfollowing substeps:

step (501), nucleating on the surface 106 and growing a plurality ofepitaxial crystal grains 1092 along the direction substantiallyperpendicular to the surface 106;

step (502), forming a continuous epitaxial film 1094 by making theepitaxial crystal grains 1092 grow along the direction substantiallyparallel to the surface 106; and

step (503), forming the first epitaxial layer 109 by making theepitaxial film 1094 grow along the direction substantially perpendicularto the surface 106.

Referring to FIGS. 18 and 19, an epitaxial structure 10 c provided inone embodiment includes a substrate 100, a first carbon nanotube layer102, and a first epitaxial layer 104, a second carbon nanotube layer107, and a second epitaxial layer 109. The substrate 100 has anepitaxial growth surface 101. The first carbon nanotube layer 102 islocated on the epitaxial growth surface 101 and defines a plurality offirst apertures 105. The first epitaxial layer 104 is located on thefirst carbon nanotube layer 102 and contacts the epitaxial growthsurface 101 through the first apertures 105. The first epitaxial layer104 defines a plurality of first caves 103. The carbon nanotubes of thefirst carbon nanotube layer 102 are enclosed in the first caves 103 andcan be spaced from the inner wall of the first caves 103.

The second carbon nanotube layer 107 is located on the surface 106 ofthe first epitaxial layer 104 and defines a plurality of secondapertures 108. The second epitaxial layer 109 is located on the secondcarbon nanotube layer 107 and contacts the surface 106 through thesecond apertures 108. The second epitaxial layer 109 defines a pluralityof second caves 110 on a surface adjacent to the first epitaxial layer104. The carbon nanotubes of the second carbon nanotube layer 107 areenclosed in the second caves 110 and covered by the first epitaxiallayer 104. The carbon nanotubes of the second carbon nanotube layer 107can be in contact with or spaced from the inner wall of the second caves110.

In one embodiment, both the first carbon nanotube layer 102 and thesecond carbon nanotube layer 107 include a drawn carbon nanotube film asshown in FIG. 18. In another embodiment, both the first carbon nanotubelayer 102 and the second carbon nanotube layer 107 of an epitaxialstructure 10 d include a layer of parallel and spaced carbon nanotubewires as shown in FIG. 20. In another embodiment, both the first carbonnanotube layer 102 and the second carbon nanotube layer 107 of anepitaxial structure 10 e include a plurality of carbon nanotube wirescrossed, or weaved together to form a carbon nanotube net as shown inFIG. 21.

Referring to FIG. 22, a method for making an epitaxial structure 20 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (30), forming an epitaxial structure preform by epitaxially growinga first epitaxial layer 104 on the epitaxial growth surface 101; and

step (60), removing the first carbon nanotube layer 102.

The method for making an epitaxial structure 20 is similar to the methodfor making the epitaxial structure 10 described above except additionalstep (60). The step (60) can be performed by plasma etching, laserheating, or furnace heating.

In one embodiment, the first carbon nanotube layer 102 is removed byplasma etching and the step (60) includes the following substeps:

step (601), placing the epitaxial structure preform in a reacting roomand creating a vacuum in the reacting room; and

step (602), introducing a reacting gas in the reacting room andproducing a plasma of the reacting gas by glow discharge.

In step (602), the reacting gas can be oxygen gas, hydrogen gas, carbontetrafluoride gas, or tetrafluoromethane gas. In one embodiment, thereacting gas is oxygen gas and oxygen plasma is produced. The plasma caninfiltrate into the first caves 103 to etch the first carbon nanotubelayer 102. The plasma can react with the first carbon nanotube layer 102from about 15 seconds to about 1 hour. The power of the glow dischargecan be in a range from about 20 watts to about 300 watts. The flow ofthe reacting gas can be in a range from about 10 sccm to about 100 sccm.The gas pressure of the reacting room is about 1 pascal to about 100pascals. In one embodiment, the reaction time is in a range from about15 seconds to about 15 minutes, the power of the glow discharge is about150 watts, and the gas pressure of the reacting room is about 10 Pa.

In one embodiment, the first carbon nanotube layer 102 is removed bylaser heating and the step (60) includes the following substeps:

step (611), placing the epitaxial structure preform in an oxygenenvironment; and

step (612), providing a laser beam to irradiate the substrate 100 or thefirst epitaxial layer 104.

In step (612), the laser beam can be provided by a laser device such asa solid laser device, a liquid laser device, a gas laser device, or asemiconductor laser device. In one embodiment, the laser device is acarbon dioxide laser device. The power of the laser device is about 30watts. The wavelength of the laser is about 10.6 micrometers. Thediameter of the laser spot is about 3 millimeters. The power density ofthe laser is about 0.053×10¹² watts per square meter. The irradiatingtime is less than 1.8 second.

The parameter of the laser should be selected according to the materialof the first epitaxial layer 104 so that the first epitaxial layer 104will not decompose. For example, if the first epitaxial layer 104includes a low-temperature GaN buffer layer and a high-temperature GaNepitaxial layer, the laser with wavelength of 248 nanometers should notbe used to heat and remove the first carbon nanotube layer 102 becausethe low-temperature GaN buffer layer can absorb the laser withwavelength of 248 nanometers and decompose to form Ga and N₂ easily.

If the substrate 100 is opaque, the substrate 100 will be heated andheat will be conducted to the first carbon nanotube layer 102. If theinner wall of the first caves 103 is spaced from the carbon nanotubes ofthe first carbon nanotube layer 102, the first caves 103 can be filledwith oxygen gas or air gas. Thus, the first carbon nanotube layer 102 iseasily oxidized. If the substrate 100 is transparent, the laser can passthrough the substrate 100 to irradiate the first carbon nanotube layer102 directly. The first carbon nanotube layer 102 can absorb the laserand oxidize easily. The laser beam can be irradiated on the epitaxialstructure preform and moved relative to the epitaxial structure preform.The laser beam can be moved along a direction parallel with orperpendicular with the aligning direction of the carbon nanotubes in thefirst carbon nanotube layer 102. The slower the laser beam movesrelative to the epitaxial structure preform, more energy will beabsorbed by the first carbon nanotube layer 102, and the shorter timethe first carbon nanotube layer 102 will oxidize. In one embodiment, thespeed of the laser beam moving relative to the epitaxial structurepreform is less than 10 millimeters per second.

Step (612) can be performed by fixing the epitaxial structure preformand moving the laser beam to irradiate the entire substrate 100. Also,step (612) can be performed by fixing the laser beam and moving theepitaxial structure preform so the entire substrate 100 is irradiated bythe laser beam.

In one embodiment, the first carbon nanotube layer 102 is removed byheating in a furnace, and the step (60) includes the following substeps:

step (621), placing the epitaxial structure preform in a furnace; and

step (622), heating the furnace to a determined temperature.

In step (621), the furnace can be any furnace according to need. In oneembodiment, the furnace is a resistance furnace filled with oxygen gasor air gas.

In step (622), the furnace is heated to a temperature above 600° C. Inone embodiment, the furnace is heated to a temperature in a range fromabout 650° C. to about 1200° C.

Referring to FIG. 23, a method for making an epitaxial structure 20 a ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101;

step (40), placing a second carbon nanotube layer 107 on a surface 106of the first epitaxial layer 104;

step (50), epitaxially growing a second epitaxial layer 109 on the firstepitaxial layer 104; and

step (60 a), removing both the first carbon nanotube layer 102 and thesecond carbon nanotube layer 107.

The method for making an epitaxial structure 20 a is similar to themethod for making an epitaxial structure 10 c described above exceptadditional step (60 a). The step (60 a) can be performed by the methodsprovided in step (60) described above.

Referring to FIG. 24, a method for making an epitaxial structure 30 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (80), forming a buffer layer 1041 on the epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the buffer layer1041;

step (30), forming an epitaxial structure preform by growing a firstepitaxial layer 104 on the buffer layer 1041; and

step (70), removing the substrate 100.

The method for making an epitaxial structure 30 is similar to the methodfor making an epitaxial structure 10 described above except additionalsteps (70) and (80).

In step (80), the buffer layer 1041 can be grown by the method ofgrowing the first epitaxial layer 104 provided in step (30) describedabove. The thickness of the buffer layer 1041 can be in a range fromabout 10 nanometers to about 50 nanometers. The material of the bufferlayer 1041 can be selected according to the material of the firstepitaxial layer 104 and the substrate 100 so that the lattice mismatchbetween the first epitaxial layer 104 and the substrate 100 can bereduced.

In step (70), the substrate 100 can be removed by laser irradiation,corrosion, or thermal expansion and contraction. The method of removingthe substrate 100 depends on the material of the buffer layer 1041, thematerial of the substrate 100, and the material of the first epitaxiallayer 104.

In one embodiment, the substrate 100 is sapphire, the buffer layer 1041is a low-temperature GaN layer, and the first epitaxial layer 104 is ahigh-temperature GaN layer. The substrate 100 is removed by laserirradiation and the step (70) includes the following substeps:

step (701), polishing and cleaning the surface of the substrate 100;

step (702), providing a laser beam to irradiate the substrate 100 andthe first epitaxial layer 104; and

step (703), placing the epitaxial structure preform in a solution.

In step (701), the surface of the substrate 100 can be polished by amechanical polishing or chemical polishing so the substrate 100 has asmooth surface to reduce the scattering in laser irradiation. Thesurface of the substrate 100 can be cleaned using hydrochloric acid orsulfuric acid to remove the metal impurities and/or oil dirt thereon.

In step (702), the epitaxial structure preform is placed on a flatsupport in a vacuum or protective gas to prevent the first carbonnanotube layer 102 from oxidation. The protective gas can be nitrogengas, helium gas, argon gas, or other inert gases.

The laser beam irradiates the polished surface of the substrate 100substantially perpendicular to the polished surface. Thus, the laserbeam can irradiate the interface between the substrate 100 and the firstepitaxial layer 104. The wavelength of the laser beam can be selectedaccording to the material of the buffer layer 1041 and the substrate 100so the energy of the laser beam is less than the band-gap energy of thesubstrate 100 and greater than the band-gap energy of the buffer layer1041. Thus, the laser beam can get through the substrate 100 to arriveat the buffer layer 1041. The buffer layer 1041 can absorb the laserbeam and be heated to decompose rapidly. In one embodiment, the bufferlayer 1041 is a low-temperature GaN layer with a band-gap energy of 3.3electron volts, the substrate 100 is sapphire with a band-gap energy of9.9 electron volts, and the laser beam has a wavelength of 248nanometers, an energy of 5 electron volts, an impulse duration fromabout 20 ns to about 40 ns, and an energy density from about 0.4 joulesper square centimeter to about 0.6 joules per square centimeter. Theshape of the laser spot is square with a side length of about 0.5millimeters. The laser spot can move relative to the substrate 100 witha speed of about 0.5 millimeters per second. After absorption of thelaser beam, the low-temperature GaN buffer layer 1041 can decompose toGa and N₂. The substrate 100 will not be damaged because only a smallamount of the laser beam is absorbed.

In step (703), the epitaxial structure preform is immersed in an acidsolution to remove the Ga decomposed from the GaN buffer layer 1041 sothe substrate 100 is separated from the first epitaxial layer 104. Theacid solution can be a hydrochloric acid, sulfuric acid, or nitric acidthat can dissolve the Ga. Because the buffer layer 1041 is locatedbetween the first carbon nanotube layer 102 and the substrate 100, thefirst carbon nanotube layer 102 will remain on the first epitaxial layer104 after the substrate 100 is separated from the first epitaxial layer104. Because the buffer layer 1041 is decomposed by laser irradiationand removed by immersing in acid solution, the first carbon nanotubelayer 102 will remain in the first caves 103. Furthermore, the N₂decomposed from the GaN buffer layer 1041 will expand and separate thefirst carbon nanotube layer 102 from the substrate 100 easily. Becausethe first carbon nanotube layer 102 allows the first epitaxial layer 104and the buffer layer 1041 to have a relative small contacting surface,the substrate 100 can be separated from the first epitaxial layer 104easily and the damage on the first epitaxial layer 104 will be reduced.

In one embodiment, the substrate 100 is SiC, the buffer layer 1041 is anAlN layer or a TiN layer, the first epitaxial layer 104 ishigh-temperature GaN layer. The substrate 100 is removed by corrodingthe buffer layer 1041 in a corrosion solution. The corrosion solutioncan dissolve the buffer layer 1041 and the substrate 100 but cannotdissolve the first epitaxial layer 104. The corrosion solution can beNaOH solution, KOH solution, or NH₄OH solution. In one embodiment, thecorrosion solution is NaOH solution with a mass concentration from about30% to about 50%. The epitaxial structure preform is immersed in theNaOH solution for about 2 minutes to about 10 minutes. The NaOH solutionenters the first caves 103 to corrode the AlN buffer layer 1041 so thesubstrate 100 is separated from the first epitaxial layer 104. If thebuffer layer 1041 is a TiN layer, the corrosion solution can be a nitricacid.

Furthermore, the substrate 100 can also be dissolved by a corrosionsolution directly. Thus, the step of growing the buffer layer 1041 canbe omitted. Because the first carbon nanotube layer 102 allows the firstepitaxial layer 104 and the buffer layer 1041 to have a relative smallcontacting surface and a plurality of first caves 103 are locatedbetween the first epitaxial layer 104 and the buffer layer 1041, thecorrosion solution can spread on the buffer layer 1041 rapidly anduniformly. Thus, the substrate 100 can be separated from the firstepitaxial layer 104 easily and the damage on the first epitaxial layer104 can be reduced.

In one embodiment, the substrate 100 is sapphire, the buffer layer 1041is a low-temperature GaN layer, and the first epitaxial layer 104 is ahigh-temperature GaN layer. The substrate 100 is removed due to thermalexpansion and contraction. The epitaxial structure preform is heated toa high temperature above 1000° C. and cooled to a low temperature below1000° C. in a short time such as from 2 minutes to about 20 minutes. Thesubstrate 100 is separated from the first epitaxial layer 104 bycracking because of the thermal expansion mismatch between the substrate100 and the first epitaxial layer 104. The epitaxial structure preformcan also be heated by applying an electrical current to the first carbonnanotube layer 102. After the epitaxial structure preform cracks, thesubstrate 100 can be removed by moving along a direction parallel withthe surface of the first carbon nanotube layer 102 so the first carbonnanotube layer 102 can remain on the first epitaxial layer 104.

Referring to FIGS. 25 and 26, an epitaxial structure 30 provided in oneembodiment includes a first epitaxial layer 104 having a patternedsurface, and a first carbon nanotube layer 102 located on the patternedsurface. The first carbon nanotube layer 102 is patterned and defines aplurality of first apertures 105 so a part of the first epitaxial layer104 protrudes from the first apertures 105. The patterned surface of theepitaxial layer 104 defines a plurality of first caves 103. The carbonnanotubes of the first carbon nanotube layer 102 are enclosed in thefirst caves 103. The first caves 103 are blind holes or grooves so apart of the first carbon nanotube layer 102 is exposed.

Furthermore, a step of removing the first carbon nanotube layer 102 canbe performed after the step (70). The first carbon nanotube layer 102can be removed by the method provided in step (60), or other methodssuch as cleaning by ultrasonic treatment, peeling by an adhesive tape,polishing by a brush, or combinations thereof.

Referring to FIG. 27, a method for making an epitaxial structure 30 a ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (80), forming a buffer layer 1041 on the epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101;

step (40), placing a second carbon nanotube layer 107 on a surface 106of the first epitaxial layer 104;

step (50), epitaxially growing a second epitaxial layer 109 on the firstepitaxial layer 104; and

step (70), removing the substrate 100.

The method for making an epitaxial structure 30 a is similar to themethod for making an epitaxial structure 10 c described above exceptthat step (80) is performed after step (10), and step (70) after step(50).

Referring to FIG. 28, a method for making an epitaxial structure 40 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (80), forming a buffer layer 1041 on the epitaxial growth surface101;

step (30), epitaxially growing a first epitaxial layer 104 on the bufferlayer 1041; and

step (70 a), removing both the substrate 100 and the first carbonnanotube layer 102.

The method for making an epitaxial structure 40 is similar to the methodfor making an epitaxial structure 30 described above except that step(80) is performed after step (20), and both the substrate 100 and thefirst carbon nanotube layer 102 are removed after step (30). Becauseboth the substrate 100 and the first carbon nanotube layer 102 areremoved after step (30), the buffer layer 1041 can be formed on theepitaxial growth surface 101 after step (20) and before step (30). Thus,the first carbon nanotube layer 102 can be attached on the substrate 100and will be removed with the substrate 100 together in step (70 a).

Referring to FIG. 29, a method for making an epitaxial structure 50 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (30), epitaxially growing a first epitaxial layer 104 on the bufferlayer 1041, wherein the first epitaxial layer 104 is an intrinsicsemiconductor epitaxial layer; and

step (90), growing a doped semiconductor epitaxial layer 112 on thefirst epitaxial layer 104.

The method for making an epitaxial structure 50 is similar to the methodfor making an epitaxial structure 10 described above except that thefirst epitaxial layer 104 is an intrinsic semiconductor epitaxial layer,and additional step (90).

In step (90), the doped semiconductor epitaxial layer 112 can be grownon the first epitaxial layer 104 by introducing a gas containing thedoping elements in the source gas for growing intrinsic semiconductorepitaxial layer. The doped semiconductor epitaxial layer 112 can be anN-type doped semiconductor epitaxial layer or a P-type dopedsemiconductor epitaxial layer. In one embodiment, the dopedsemiconductor epitaxial layer 112 includes an N-type doped semiconductorepitaxial layer 1120 and a P-type doped semiconductor epitaxial layer1122 to form a PN junction. In one embodiment, an active layer (notshown) can be formed between the N-type doped semiconductor epitaxiallayer 1120 and the P-type doped semiconductor epitaxial layer 1122. Theactive layer can be a single-layer quantum well structure ormultiple-layer quantum well structure. In one embodiment, a highly dopedsemiconductor electrode contacting layer (not shown) can be formed on asurface of the PN junction away from the substrate 100. Furthermore, aprocess of annealing the doped semiconductor epitaxial layer 112 can beperformed to activate the doping elements of the doped semiconductorepitaxial layer 112.

In another embodiment, the gas containing the doping elements isintroduced in the source gas for growing the first epitaxial layer 104,and a doped semiconductor epitaxial layer can be grown on the epitaxialgrowth surface 101 directly.

In another embodiment, if the first epitaxial layer 104 is an intrinsicsemiconductor epitaxial layer, a step of forming a doped semiconductorepitaxial layer can be performed after the step (30) by doping theintrinsic semiconductor epitaxial layer. The intrinsic semiconductorepitaxial layer can be doped by thermal diffusion or ion implantation.

Referring to FIGS. 30 and 31, an epitaxial structure 50 provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,and a first epitaxial layer 104, and a doped semiconductor epitaxiallayer 112. The epitaxial structure 50 is similar to the epitaxialstructure 10 described above except that the first epitaxial layer 104is an intrinsic semiconductor epitaxial layer and a doped semiconductorepitaxial layer 112 is formed on the intrinsic semiconductor epitaxiallayer. The doped semiconductor epitaxial layer 112 includes an N-typedoped semiconductor epitaxial layer 1120 and a P-type dopedsemiconductor epitaxial layer 1122 to form a PN junction. In oneembodiment, an active layer (not shown) can be formed between the N-typedoped semiconductor epitaxial layer 1120 and the P-type dopedsemiconductor epitaxial layer 1122. The active layer can be asingle-layer quantum well structure or multiple-layer quantum wellstructure. Also, a highly doped semiconductor electrode contacting layer(not shown) can be formed on a surface of the PN junction away from thesubstrate 100. In one embodiment, the intrinsic semiconductor epitaxiallayer 104 can be omitted, and the doped semiconductor epitaxial layer112 can be located on and contacting the epitaxial growth surface 101.

Referring to FIG. 32, a method for making an epitaxial structure 60 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20 a), suspending a first carbon nanotube layer 102 above theepitaxial growth surface 101; and

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101.

The method for making an epitaxial structure 60 is similar to the methodfor making an epitaxial structure 10 described above except that in step(20 a), the first carbon nanotube layer 102 is suspended above theepitaxial growth surface 101.

In step (20 a), the first carbon nanotube layer 102 is a free-standingstructure. The first carbon nanotube layer 102 is spaced from andextends along a direction parallel with the epitaxial growth surface101. The first carbon nanotube layer 102 can cover the entire epitaxialgrowth surface 101 or have an area smaller than the area of theepitaxial growth surface 101. The carbon nanotubes of the first carbonnanotube layer 102 are arranged along a direction substantially parallelwith the epitaxial growth surface 101. The distance between the firstcarbon nanotube layer 102 and the epitaxial growth surface 101 can beselected according to need, such as in a range from about 10 nanometersto about 500 micrometers. In one embodiment, the distance between thefirst carbon nanotube layer 102 and the epitaxial growth surface 101 isin a range from about 50 nanometers to about 100 micrometers, such asabout 10 micrometers. Thus, the first epitaxial layer 104 can growthrough the first apertures 105 of the first carbon nanotube layer 102and enclose the first carbon nanotube layer 102 therein. The firstcarbon nanotube layer 102 can prevent a part of the first epitaxiallayer 104 from epitaxially growing vertically and cause the firstepitaxial layer 10 to lateral epitaxially grow laterally. Thus, thelattice mismatch between the substrate 100 and the first epitaxial layer104 can be reduced and the quality of the first epitaxial layer 104 canbe improved.

In one embodiment, the step (20 a) includes the following substeps:

step (201), providing a first support 114 and a second support 116 andplacing the first support 114 and the second support 116 spaced fromeach other;

step (202), placing the substrate 100 between the first support 114 andthe second support 116; and

step (203), placing the first carbon nanotube layer 102 on the firstsupport 114 and the second support 116.

In step (201), the first support 114 and the second support 116 can bemade of a material such as metal, alloy, polymer, glass, or ceramic. Thedistance between the first support 114 and the second support 116 can beselected according to need. In one embodiment, the distance between thefirst support 114 and the second support 116 is greater than the size ofthe substrate 100 so the suspended first carbon nanotube layer 102 cancover the entire epitaxial growth surface 101.

In step (202), the height of the first support 114 and the secondsupport 116 is higher than the thickness of the substrate 100.

In step (203), one side of the first carbon nanotube layer 102 can beattached on the first support 114 and the other opposite side can beattached on the second support 116. The part of the first carbonnanotube layer 102 between the first support 114 and the second support116 is tightened and suspended. The carbon nanotubes of the first carbonnanotube layer 102 are arranged to extend from the first support 114 tothe second support 116. The first carbon nanotube layer 102 can be fixedon the first support 114 and the second support 116 by a conductiveadhesive.

In step (30), the first epitaxial layer 104 starts growing from theepitaxial growth surface 101. When the first epitaxial layer 104 growsto the first carbon nanotube layer 102, the first epitaxial layer 104can only grow vertically through the first apertures 105 of the firstcarbon nanotube layer 102 and then laterally grows and joins together toenclose the first carbon nanotube layer 102 therein. Thus, a pluralityof first caves 103 is formed in the first epitaxial layer 104. The firstcaves 103 can join together to form a pattern same as the pattern of thefirst carbon nanotube layer 102. Furthermore, a voltage can be suppliedto between the first support 114 and the second support 116 so that thefirst carbon nanotube layer 102 can produce heat to heat the substrate100. Thus, the quality of the first epitaxial layer 104 can be improved.

Referring to FIG. 33, an epitaxial structure 60 provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,and a first epitaxial layer 104. The epitaxial structure 30 is similarto the epitaxial structure 10 described above except that the firstcarbon nanotube layer 102 is located in and enclosed by the firstepitaxial layer 104. The first epitaxial layer 104 defines a pluralityof first caves 103 therein. The first caves 103 are arranged in a plane.The carbon nanotubes of the first carbon nanotube layer 102 are locatedin the first caves 103. If the first carbon nanotube layer 102 includesa layer of parallel and spaced carbon nanotube wires, the first caves103 are a plurality of parallel and spaced tunnels. If the first carbonnanotube layer 102 includes a plurality of carbon nanotube wires crossedor weaved together to form a carbon nanotube net, the first caves 103are a plurality of intersected tunnels interconnected with each other.The cross section of the tunnel can be round with a diameter in a rangefrom about 2 nanometers to about 200 micrometers. In one embodiment, thediameter of the tunnel is in a range from about 2 nanometers to about200 nanometers.

Furthermore, as shown in FIG. 34, in one embodiment, the two firstcarbon nanotube layers 102 are suspended above the epitaxial growthsurface 101 in step (20 a). The two first carbon nanotube layers 102 areparallel with and spaced from each other. The distance between the twofirst carbon nanotube layers 102 is in a range from about 10 nanometersto about 500 micrometers. Also, more than two first carbon nanotubelayers 102 can be suspended above the epitaxial growth surface 101. Themore than two first carbon nanotube layers 102 can be suspendedequidistantly. As shown in FIG. 35, an epitaxial structure 60 a providedin one embodiment includes a substrate 100, a first epitaxial layer 104located on the substrate 100, and two first carbon nanotube layers 102located in the first epitaxial layer 104 and spaced from each other. Theepitaxial structure 30 can also include more than two first carbonnanotube layers 102 located in the first epitaxial layer 104equidistantly.

Referring to FIG. 36, a method for making an epitaxial structure 70 ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101; and

step (30 a), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101, wherein the first epitaxial layer 104 onlyincludes a plurality of epitaxial crystal grains 1042 spaced from eachother.

The method for making an epitaxial structure 70 is similar to the methodfor making an epitaxial structure 10 described above except that in step(30 a), the plurality of epitaxial crystal grains 1042 are not joinedtogether to form an continuous integral structure to cover the firstcarbon nanotube layer 102.

In step (30 a), the epitaxial crystal grains 1042 grow substantiallyvertically from the exposed epitaxial growth surface 101 and through thefirst apertures 105. The thickness of the first epitaxial layer 104 canbe controlled by controlling the growing time so that a plurality ofspaced epitaxial crystal grains 1042 can be obtained, not a continuousfilm. The plurality of epitaxial crystal grains 1042 define a patternedspace there between. The first carbon nanotube layer 102 is located inthe patterned space. The patterned space has the same pattern as thepatterned first carbon nanotube layer 102. If the first carbon nanotubelayer 102 includes a layer of substantially parallel and spaced carbonnanotube wires, the patterned space is a plurality of substantiallyparallel and spaced grooves. If the first carbon nanotube layer 102includes a plurality of carbon nanotube wires crossed or weaved togetherto form a carbon nanotube net, the patterned space is a plurality ofintersected grooves.

Furthermore, a step of removing the first carbon nanotube layer 102 canbe performed after the step (30 a). The first carbon nanotube layer 102can be removed by the method provided in step (60), or other methodssuch as peeling by ultrasonic treatment, peeling by an adhesive tape,polishing by a brush, or combinations thereof.

Referring to FIG. 37, an epitaxial structure 70 provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,and a first epitaxial layer 104. The first epitaxial layer 104 includesa plurality of epitaxial crystal grains 1042 spaced from each other anddefines a patterned space. The first carbon nanotube layer 102 islocated in the patterned space. The patterned space has the same patternas the patterned first carbon nanotube layer 102. The shape of theepitaxial crystal grains 1042 depends on the shape of the first openings105. If the first opening 105 is a round hole, the epitaxial crystalgrains 1042 can be a cylinder. If the first opening 105 is a gap, theepitaxial crystal grains 1042 can be cuboid.

Referring to FIG. 38, a method for making an epitaxial structure 70 a ofone embodiment includes the following steps:

step (10), providing a substrate 100 having an epitaxial growth surface101;

step (20), placing a first carbon nanotube layer 102 on the epitaxialgrowth surface 101;

step (80), forming a buffer layer 1041 on the epitaxial growth surface101;

step (30), epitaxially growing a first epitaxial layer 104 on theepitaxial growth surface 101;

step (40), placing a second carbon nanotube layer 107 on a surface 106of the first epitaxial layer 104; and

step (50 a), epitaxially growing a second epitaxial layer 109 on thefirst epitaxial layer 104, wherein the second epitaxial layer 109 onlyincludes a plurality of epitaxial crystal grains 1092 spaced from eachother.

The method for making an epitaxial structure 70 a is similar to themethod for making an epitaxial structure 20 a described above exceptthat a step (80) of forming a buffer layer 1041 on the epitaxial growthsurface 101 is performed after step (20) and before step (30), and instep (50 a) the plurality of epitaxial crystal grains 1092 are notjoined together to form an continuous integral structure to cover thefirst carbon nanotube layer 102.

Furthermore, a step of removing the second carbon nanotube layer 107 canbe performed after the step (50 a) to obtain an epitaxial structure 70 bas shown in FIG. 38. The second carbon nanotube layer 107 can be removedby the method provided in step (60), or other methods such as peeling byultrasonic treatment, peeling by an adhesive tape, polishing by a brush,or combinations thereof.

Furthermore, a step of removing the substrate 100 and the first carbonnanotube layer 102 can be performed to obtain an epitaxial structure 70c as shown in FIG. 38. The substrate 100 can be removed by the methodprovided in step (70). The first carbon nanotube layer 102 can beremoved by the method provided in step (60). Also the substrate 100 andthe first carbon nanotube layer 102 can be removed together.

Referring to FIG. 39, an epitaxial structure 70 a provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,a first epitaxial layer 104, a second carbon nanotube layer 107, and asecond epitaxial layer 109. The epitaxial structure 70 a is similar tothe epitaxial structure 10 c described above except that the secondepitaxial layer 109 includes a plurality of epitaxial crystal grains1092 spaced from each other and defines a patterned space, and thesecond carbon nanotube layer 107 is located in the patterned space. Thepatterned space has the same pattern as the patterned second carbonnanotube layer 107. Furthermore, a buffer layer 1041 can be locatedbetween the substrate 100 and the first epitaxial layer 104. The bufferlayer 1041 is located on the substrate 100 and in the first apertures105 of the first carbon nanotube layer 102.

Referring to FIG. 40, an epitaxial structure 70 b provided in oneembodiment includes a substrate 100, a first carbon nanotube layer 102,a first epitaxial layer 104, and a second epitaxial layer 109. Theepitaxial structure 70 b is similar to the epitaxial structure 70 adescribed above except that no carbon nanotube layer is located in thepatterned space.

The following examples are provided to more particularly illustrate thedisclosure, and should not be construed as limiting the scope of thedisclosure.

Example 1

In example 1, the substrate is a SOI, the epitaxial layer is a GaN layerand grown on the SOI substrate by a MOCVD method. The nitrogen sourcegas is high-purity ammonia (NH₃), the Ga source gas is trimethyl gallium(TMGa) or triethyl gallium (TEGa), and the carrier gas is hydrogen (H₂).A single drawn carbon nanotube film is placed on an epitaxial growthsurface of the SOI substrate. The growth of the epitaxial layer includesthe following steps:

step (a), putting the SOI substrate with the drawn carbon nanotube filmthereon into a vacuum reaction chamber and heating the reaction chamberto a temperature of about 1070° C.;

step (b), introducing the nitrogen source gas and the Ga source gas intothe vacuum reaction chamber with the carrier gas;

step (c), vertical epitaxially growing a plurality of GaN epitaxialgrains for about 450 seconds at about 1070° C.;

step (d), heating the reaction chamber to about 1110° C., reducing theflow of the Ga source gas, keeping the gas pressure of the reactionchamber and the flow of the nitrogen source gas unchanged, and makingthe GaN epitaxial grains epitaxially grow laterally for about 4900seconds at about 1110° C. to obtain a GaN epitaxial film;

step (e), cooing the temperature of the reaction chamber down to about1070° C., and increasing the flow of the Ga source gas, and making theGaN epitaxial film epitaxially grow vertically for about 10000 secondsat about 1070° C. to form a GaN epitaxial layer.

The epitaxial structure provided in example 1 is observed by SEM andTEM. Referring to FIGS. 41 and 42, the dark-colored layer is theepitaxial layer, and the light-colored layer is the substrate. Aplurality of grooves is defined on the face of the epitaxial layer. Thegrooves are covered by the substrate to form a plurality of tunnels. Thecarbon nanotubes are located in the tunnels.

Example 2

In example 2, the substrate is sapphire, the epitaxial layer is grown onthe sapphire substrate by MOCVD method. The nitrogen source gas ishigh-purity ammonia (NH₃), the Ga source gas is trimethyl gallium (TMGa)or triethyl gallium (TEGa), and the carrier gas is hydrogen (H₂). Asingle drawn carbon nanotube film is placed on an epitaxial growthsurface of the sapphire substrate. The growth of the epitaxial layerincludes the following steps:

step (a), locating the sapphire substrate with the single drawn carbonnanotube film thereon into a reaction chamber, heating the sapphiresubstrate to about 1100° C. to about 1200° C., introducing the carriergas, and baking the sapphire substrate for about 200 seconds to about1000 seconds;

step (b), growing a low-temperature GaN buffer layer with a thickness ofabout 10 nanometers to about 50 nanometers by cooling down thetemperature of the reaction chamber to a range from about 500° C. to650° C. in the carrier gas atmosphere, and introducing the Ga source gasand the nitrogen source gas at the same time;

step (c), stopping the flow of the Ga source gas while maintaining theflow of the carrier gas and nitrogen source gas atmosphere, increasingthe temperature to a range from about 1100° C. to about 1200° C., andannealing for about 30 seconds to about 300 seconds; and

step (d), maintaining the temperature of the reaction chamber in a rangefrom about 1000° C. to about 1100° C., and reintroducing the Ga sourcegas to grow the high quality epitaxial layer.

Furthermore, the epitaxial structure provided in example 2 is observedby SEM and TEM. Referring to FIGS. 43 and 44, the dark-colored layer isthe GaN epitaxial layer, and the light-colored layer is the sapphiresubstrate. A plurality of grooves is defined on the face of the GaNepitaxial layer. The grooves are covered by the sapphire substrate toform a plurality of tunnels. The carbon nanotubes are located in thetunnels.

Example 3

Example 3 is similar to example 2 described above except that a step (e)of irradiating the epitaxial structure with a laser beam in air isperformed after step (d). In step (e), the drawn carbon nanotube film isremoved by oxidation. The laser beam is provided by a carbon dioxidelaser device. The power of the laser device is about 30 watts. Thewavelength of the laser is about 10.6 micrometers. The diameter of thelaser spot is about 3 millimeters. The power density of the laser isabout 0.053×10¹² watts per square meter. The irradiating time is lessthan 1.8 seconds.

Example 4

In example 4, the substrate is sapphire, the epitaxial layer is grown onthe sapphire substrate by MOCVD method. The nitrogen source gas ishigh-purity ammonia (NH₃), the Ga source gas is trimethyl gallium (TMGa)or triethyl gallium (TEGa), and the carrier gas is hydrogen (H₂). Asingle drawn carbon nanotube film is placed on an epitaxial growthsurface of the sapphire substrate. The method of making the epitaxialstructure includes the following steps:

step (a), locating the sapphire substrate into a reaction chamber,heating the sapphire substrate to about 1100° C. to about 1200° C.,introducing the carrier gas, and baking the sapphire substrate for about200 seconds to about 1000 seconds;

step (b), growing a low-temperature GaN buffer layer with a thickness ofabout 10 nanometers to about 50 nanometers by cooling down thetemperature of the reaction chamber to a range from about 500° C. to650° C. in the carrier gas atmosphere, and introducing the Ga source gasand the nitrogen source gas at the same time;

step (c), stopping the flow of the Ga source gas, while maintaining theflow of the carrier gas and nitrogen source gas atmosphere, increasingthe temperature to a range from about 1100° C. to about 1200° C., andannealing for about 30 seconds to about 300 seconds;

step (d), placing a single drawn carbon nanotube film on thelow-temperature GaN buffer layer;

step (e), maintaining the temperature of the reaction chamber in a rangefrom about 1000° C. to about 1100° C., and reintroducing the Ga sourcegas to grow the high quality epitaxial layer; and

step (f), irradiating the epitaxial structure with a laser beam invacuum.

In step (f), the laser beam has a wavelength of about 248 nanometers, anenergy of about 5 electron volts, an impulse duration from about 20 nsto about 40 ns, an energy density from about 0.4 joules per squarecentimeter to about 0.6 joules per square centimeter. The shape of thelaser spot is square with a side length of about 0.5 millimeters. Thelaser spot moves relative to the substrate with a speed of about 0.5millimeters per second. After absorption of the laser beam, thelow-temperature GaN buffer layer is decomposed to Ga and N₂. Theepitaxial structure is immersed in a hydrochloric acid solution toremove the Ga and separate the substrate from the epitaxial layer, withthe drawn carbon nanotube film remaining on the epitaxial layer.

Example 5

In example 5, the substrate is sapphire, the epitaxial layer is grown onthe sapphire substrate by a MOCVD method. The nitrogen source gas ishigh-purity ammonia (NH₃), the Ga source gas is trimethyl gallium (TMGa)or triethyl gallium (TEGa), the carrier gas is hydrogen (H2), the Insource gas is Trimethyl indium (TMIn), the Si source gas is silane(SiH₄), and the Mg source gas is ferrocene magnesium (Cp₂Mg). A singledrawn carbon nanotube film is placed on an epitaxial growth surface ofthe sapphire substrate. The method of making the epitaxial structureincludes the following steps:

step (a), locating the sapphire substrate with a single drawn carbonnanotube film thereon into a reaction chamber, heating the sapphiresubstrate to about 1100° C. to about 1200° C., introducing the carriergas, and baking the sapphire substrate for about 200 seconds to about1000 seconds;

step (b), growing the low-temperature GaN buffer layer with a thicknessof about 10 nanometers to about 50 nanometers by cooling down thetemperature of the reaction chamber to a range from about 500° C. to650° C. in the carrier gas atmosphere, maintaining the chamber at apressure from about 500 torr to about 600 torr, and introducing the Gasource gas and the nitrogen source gas at the same time;

step (c), stopping the flow of the Ga source gas, while maintaining theflow of the carrier gas and nitrogen source gas atmosphere, increasingthe temperature to a range from about 1100° C. to about 1200° C., thepressure to a range from about 1100 torr to about 1200 torr, andannealing for about 30 seconds to about 300 seconds;

step (d), growing a Si doped N-type GaN epitaxial layer with a thicknessof about 1 micrometer to about 3 micrometers by maintaining thetemperature of the reaction chamber in a range from about 1000° C. toabout 1100° C. at a pressure from about 100 torr to about 300 torr,introducing the Ga source gas and the Si source gas to;

step (e), growing a InGaN/GaN multiple-layer quantum well by stoppingthe flow of the Si source gas, maintaining the chamber in a temperaturefrom about 700° C. to about 900° C. at a pressure from about 50 torr toabout 500 torr, and introducing the In source gas, wherein the InGaNlayer has a thickness of about 2 nanometers to about 5 nanometers, andthe GaN layer has a thickness of about 5 nanometers to about 20nanometers;

step (f), grow a Mg doped P-type GaN epitaxial layer with a thickness ofabout 100 nanometers to about 200 nanometers by stopping the flow of theIn source gas, maintaining the chamber in a temperature from about 1000°C. to about 1100° C. at a pressure from about 76 torr to about 200 torr,and introducing the Mg source gas; and

step (g), stopping growth, introducing N₂ gas, and maintaining thechamber in a temperature from about 700° C. to about 800° C. to annealfor about 10 minutes to about 20 minutes.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making an epitaxial structure, themethod comprising: providing a substrate having an epitaxial growthsurface; placing a first carbon nanotube layer on the epitaxial growthsurface; epitaxially growing a first epitaxial layer on the epitaxialgrowth surface; placing a second carbon nanotube layer on a surface ofthe first epitaxial layer; and epitaxially growing a second epitaxiallayer on the surface of the first epitaxial layer.
 2. The method ofclaim 1, wherein a material of the substrate is GaAs, GaN, AlN, Si, SOI,SiC, MgO, ZnO, LiGaO2, LiAlO2, or Al2O3.
 3. The method of claim 1,wherein the first carbon nanotube layer is free-standing and laid on theepitaxial growth surface directly.
 4. The method of claim 1, wherein thefirst carbon nanotube layer defines a plurality of first apertures toexpose a part of the epitaxial growth surface, and the first epitaxiallayer is grown from the exposed part of the epitaxial growth surface andthrough the apertures.
 5. The method of claim 4, wherein sizes of thefirst apertures are in a range from about 10 nanometers to about 500micrometers.
 6. The method of claim 4, wherein a dutyfactor of the firstcarbon nanotube layer is in a range from about 1:4 to about 4:1.
 7. Themethod of claim 4, wherein the step of epitaxially growing the firstepitaxial layer on the epitaxial growth surface comprises: nucleating onthe epitaxial growth surface and growing a plurality of epitaxialcrystal grains along a direction substantially perpendicular to theepitaxial growth surface; forming a continuous epitaxial film by growingthe epitaxial crystal grains along a direction substantially parallel tothe epitaxial growth surface; and forming the first epitaxial layer bygrowing the epitaxial film along the direction substantiallyperpendicular to the epitaxial growth surface.
 8. The method of claim 7,wherein the epitaxial crystal grains grow and forms a plurality of firstcaves to enclose the first carbon nanotube layer so that the epitaxialfilm defines a patterned depression on a surface adjacent to theepitaxial growth surface.
 9. The method of claim 8, wherein the firstcarbon nanotube layer comprises a layer of substantially parallel andspaced carbon nanotube wires, and the patterned depression is aplurality of substantially parallel and spaced grooves.
 10. The methodof claim 8, wherein the first carbon nanotube layer comprises aplurality of carbon nanotube wires crossed or weaved together to form acarbon nanotube net, and the patterned depression is a groove networkcomprising a plurality of intersected grooves.
 11. The method of claim1, wherein the first carbon nanotube layer comprises a plurality ofcarbon nanotubes extending along a direction substantially parallel tothe epitaxial growth surface.
 12. The method of claim 1, wherein thefirst carbon nanotube layer comprises a plurality of carbon nanotubesextending along a crystallographic orientation of the substrate.
 13. Themethod of claim 1, wherein the first carbon nanotube layer comprises acarbon nanotube matrix and non-carbon nanotube materials.
 14. The methodof claim 1, wherein the second carbon nanotube layer defines a pluralityof second apertures to expose a part of the surface of first epitaxiallayer, and the second epitaxial layer is grown from the exposed part ofthe surface of the first epitaxial layer and through the secondapertures.
 15. The method of claim 14, wherein a plurality of secondcaves is formed on a surface of the second epitaxial layer adjacent tothe first epitaxial layer, and the second carbon nanotube layer isenclosed in the plurality of second caves.
 16. The method of claim 1,wherein the second carbon nanotube layer is suspended above the surfaceof first epitaxial layer.
 17. The method of claim 1, further comprisinga step of forming a buffer layer on the epitaxial growth surface beforeplacing the first carbon nanotube layer, and the first carbon nanotubelayer is placed on the buffer layer.
 18. The method of claim 1, furthercomprising a step of removing the substrate.
 19. The method of claim 1,further comprising a step of removing the first carbon nanotube layer.20. The method of claim 1, further comprising a step of removing thesecond carbon nanotube layer.